niter: Make build_cltz_expr more robust [PR118689]

Since my r15-7223 the niter analysis can recognize one loop during bootstrap
as being ctz like.
The patch just turned
@@ -2173,7 +2173,7 @@ PROC m2pim_NumberIO_BinToStr (CARDINAL x
   _T535_44 = &buf[i.40_2]{lb: 1 sz: 4};
   _T536_45 = x_21 & 1;
   *_T535_44 = _T536_45;
-  _T537_47 = x_21 / 2;
+  _T537_47 = x_21 >> 1;
   x_48 = _T537_47;
   # DEBUG x => x_48
   if (x_48 != 0)
which is not a big deal for the number_of_iterations_cltz optimization, it
recognizes both right shift by 1 and unsigned division by 2 (and similarly
for clz left shift by 1 or multiplication by 2).
But starting with forwprop1 that change also resulted in
@@ -1875,9 +1875,9 @@ PROC m2pim_NumberIO_BinToStr (CARDINAL x
   i.40_2 = (INTEGER) _T530_34;
   _T536_45 = x_21 & 1;
   MEM <CARDINAL[1:64]> [(CARDINAL *)&buf][i.40_2]{lb: 1 sz: 4} = _T536_45;
-  _T537_47 = x_21 / 2;
+  _T537_47 = x_21 >> 1;
   # DEBUG x => _T537_47
-  if (x_21 > 1)
+  if (_T537_47 != 0)
     goto <bb 3>; [INV]
   else
     goto <bb 8>; [INV]
and apparently it is only the latter form that number_of_iterations_cltz
pattern matches, not the former (after all, that was the exact reason
for r15-7223).
The problem is that build_cltz_expr assumes if IFN_C[LT]Z can't be used it
can use the __builtin_c[lt]z{,l,ll} builtins, and while most of the FEs do
create them, modula 2 does not.

The following patch just lets us punt if the FE doesn't build those builtins.
I've filed a PR against modula2 so that they add the builtins too.

2025-01-31  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/118689
	PR modula2/115032
	* tree-ssa-loop-niter.cc (build_cltz_expr): Return NULL_TREE if fn is
	NULL and use_ifn is false.